Secure peer to peer coin flip over WebRTC

p2p-coinflip.js

#!/usr/bin/env node
// dependencies: npm install --global wrtc
// usage: node p2p-coinflip.js [offer?]

const crypto = require('crypto')
const readline = require('readline')
const zlib = require('zlib')

const {
  RTCPeerConnection,
  RTCSessionDescription 
} = require('/usr/local/lib/node_modules/wrtc')

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
})

const iceServers = [
  { url: 'stun:stun.l.google.com:19302' }
]

let pc, channel

const pcSettings = [
  { iceServers },
  { optional: [{ DtlsSrtpKeyAgreement: false }] }
]

let isOffering = false

const state = {
  choice: crypto.randomBytes(32),
  hash: null
}

const makeOffer = () => {
  isOffering = true
  pc = new RTCPeerConnection(pcSettings)
  channel = pc.createDataChannel('p2choice')
  handleDataChannel() 
  pc.createOffer((description) => {
    setLocalDescription(description)
  }, errorHandler)
  pc.onicecandidate = (({ candidate }) => {
    if (!candidate) {
      const offerPacket = zlib.gzipSync(JSON.stringify(pc.localDescription))
      console.log('offer:', offerPacket.toString('hex'))
      rl.question('answer: ', (answerPacket) => {
        const answer = zlib.gunzipSync(new Buffer(answerPacket, 'hex'))
        setAnswer(answer)
      })
    }
  })
}

const setOffer = (offerJSON) => {
  const offer = new RTCSessionDescription(JSON.parse(offerJSON))
  pc = new RTCPeerConnection(pcSettings)
  pc.onicecandidate = ({ candidate }) => {
    if (!candidate) {
      const answerPacket = zlib.gzipSync(JSON.stringify(pc.localDescription))
      console.log('answer:', answerPacket.toString('hex'))
    }
  }
  pc.ondatachannel = (event) => {
    channel = event.channel
    channel.binaryType = 'arraybuffer'
    handleDataChannel()
  }
  pc.setRemoteDescription(offer, handleCreateAnswer, errorHandler)
}

const setLocalDescription = (description) => {
  pc.setLocalDescription(description, noHandler, errorHandler)
}

const setAnswer = (answerJSON) => {
  pc.setRemoteDescription(new RTCSessionDescription(JSON.parse(answerJSON)))
}

const handleCreateAnswer = () => {
  pc.createAnswer(setLocalDescription, errorHandler)
}

const handleDataChannel = () => {
  channel.onopen = () => {
    //setTimeout(() => win('timeout'), 1000)
    if (isOffering) {
      sendChoiceHash()
    }
  }
  channel.onclose = () => win('remote closed')
  channel.onmessage = messageHandler
  channel.onerror = errorHandler
}

const noHandler = () => null

const messageHandler = ({ data }) => {
  if (!state.hash) {
    state.hash = new Buffer(data)
    if (!isOffering) sendChoiceHash()
    else sendChoice()
  } else {
    if (!isOffering) sendChoice()
    checkHash(data)
  }
}

const errorHandler = (err) => {
  console.error(err)
  process.exit(1)
}

const sendChoiceHash = () => {
  const hash = crypto.createHash('sha256')
  hash.update(state.choice)
  channel.send(hash.digest())
}

const sendChoice = () => {
  channel.send(state.choice)
}

const sendReplayChoiceHash = () => {
  channel.send(state.hash)
}

const sendReplayChoice = (data) => {
  state.choice = data
  channel.send(data)
}

const checkHash = (choice) => {
  const hash = crypto.createHash('sha256')
  choice = new Buffer(choice)
  hash.update(choice)
  const digest = hash.digest()
  if (digest.equals(state.hash)) {
    const ours = toBitArray(state.choice)
      .reduce((r, b) => r ^ b, 0)
    const theirs = toBitArray(choice) 
      .reduce((r, b) => r ^ b, 0)
    if (isOffering) {
      if (ours === theirs) win()
      else lose()
    } else {
      if (ours !== theirs) win()
      else lose()
    }
  } else {
    console.log('remote cheated:', 
      `sha256(${choice.toString('hex')}) should be`,
      `${digest.toString('hex')},`,
      `instead it was ${state.hash.toString('hex')}`)
    win()
  }
}

const toBitArray = (x) => {
  let bits = []
  let tmp = x
  if (typeof x === 'number') {
    while (tmp > 0) {
      bits.push(tmp % 2)
      tmp = Math.floor(tmp / 2)
    }
    return bits.reverse()
  }
  for (var i = 0; i < x.length; i++) {
    bits = bits.concat(toBitArray(x[i]))
  }
  return bits
}

const win = (info) => {
  console.log(`${info ? info + ', ' : ''}you won`)
  process.exit(0)
}

const lose = (info) => {
  console.log(`${info ? info + ', ' : ''}you lost`)
  process.exit(1)
}

if (!process.argv[2]) {
  makeOffer()
} else {
  const offer = zlib.gunzipSync(new Buffer(process.argv[2], 'hex'))
  setOffer(offer)
}